It is known to use diffractometry to detect certain crystalline substances such as most explosives or numerous other dangerous or illegal structures. Within a crystal, which is an arrangement of atoms, elastically scattered electromagnetic waves interfere with each other to give scattering which is coherent at the scale of the crystal. When those interferences are constructive, they may be detected by the measurement of a diffracted ray and by the identification of the diffraction peaks. Thus, the constructive interferences are located by an appearance of diffraction peaks (or Bragg peaks) in the radiation diffused by a material.
To know whether a given crystalline substance is contained in a material, it is thus known to:                irradiate a sample of the material using an incident beam with a central axis X, emitted by a source,        study the diffracted radiation using a detection device comprising                    a detector, here termed spectrometric detector, configured to establish an energy spectrum of the radiation diffused at a given scatter angle, that is to say a detector comprising                        a detector material, which, on the near side to the sample of materials, presents a plane here termed detection plane,        means, here termed spectrometry measurement means, configured to measure an energy released by each interaction of a photon with the detector material and to establish at least one energy spectrum, denoted S(E).        a collimator, here termed detection collimator, associated with the detector, the detector and the detection collimator being arranged so as to have a detection axis D, the detection axis D forming a diffraction angle θ with the central axis X of the incident beam.        
It is to be noted that an energy spectrum illustrates the energy distribution of radiation in the form of a histogram representing the number of photon interactions in the material (along the y-axis) according to the released energy (along the x-axis). Generally, the energy axis is discretized into channels of width 2 δE, a channel Ci corresponding to the energies comprised between Ei−δE and Ei+δE.
The various peaks obtained on an energy spectrum of a radiation that is scattered, at an angle θ, are characteristic of the material analyzed, since the scattered radiation participating in the constructive interferences satisfies the following equation:
      E    hkl    =      n    ⁢          hc              2        ⁢                                  ⁢                  d          hkl                ⁢                  sin          ⁡                      (                          θ              /              2                        )                              
with:    dhkl: interplanar spacing between the crystallographic planes of the irradiated crystal;    θ: scatter angle, that is to say the angle formed between the scattered ray analyzed and the beam that is incident on the irradiated crystal    h: Planck's constant    c: the speed of light    n: the order of the interference:
This property is exploited in well-known methods, designated by the acronym EDXRD or “Energy Dispersive X-Ray Diffraction”.
WO2008/142446 describes a method of determining the composition of an object by the spectrometric detection of an object irradiated by x-ray radiation. In the description of the prior art of WO2008/142446, reference is made to baggage checking. The method described comprises the following steps:
irradiating the object, particularly for example using x-ray radiation,
detecting the intensity transmitted through the object using a spectrometric detector. It is to be noted that the radiation studied here is the radiation transmitted by the sample of materials and is not diffracted; in other words, the detection axis D coincides with the axis X of the incident beam,
selecting energy bands in the transmitted spectrum, and establishing transmission quantities in each of those bands,
comparing at least two of said obtained quantities.
According to a first embodiment, it is sought to identify the material by detecting a Bragg detection signature. For this, a discontinuity in the transmitted intensity is revealed, at a given energy, or at least within a narrow energy band. This discontinuity is assumed to correspond to a localized drop in the amplitude of the transmitted signal under the effect of elastic scattering (Bragg diffraction) in the crystal lattice of the material analyzed. This scattering only occurs for certain discrete incident energies Ei and it is considered that in the neighborhood of that energy, the transmitted signal decreases. Thus, by comparing the intensity of the signal transmitted at that energy in a narrow energy band centered on Ei with the signal transmitted at another energy, the presence of a particular material is detected. In other words, in this application, analysis is made of the radiation transmitted by the object, and in particular of the discontinuities in its energy spectrum on account of Bragg diffraction.
In diffractometry, two parameters are essential to obtain a reliable and effective detection system: sensitivity and energy resolution. As always, these two parameters vary inversely: the improvement of one is accompanied by the degradation of the other. In a previous application FR11/62497 the inventors provided a method and device for analyzing a sample of materials by diffractometry, of which the energy resolution is improved, in order to obtain clearer separation of the diffraction peaks for a better identification of materials. In that earlier method:                a diffractometer is used which comprises                    a source configured to emit an incident beam with a central axis X,            a detector comprising                        a detector material, which, on the near side to the sample of materials, presents a plane here termed detection plane,        means, here termed spectrometry measurement means, configured to measure an energy released by each photon interaction with the detector material and to establish at least one energy spectrum, denoted S(E), the detector being here termed spectrometric detector on account of this;        means for locating an interaction of a photon with the detector material, making it possible to define a partition of the detector in physical or virtual pixels, and making it possible to associate one of said pixels with each photon interaction; the detector used being here termed a pixelated detector on account of this;                    a collimator, here termed detection collimator, associated with the detector, the detector and the detection collimator being arranged so as to have a detection axis D forming a diffraction angle θ with the central axis X of the incident beam,                        the sample is irradiated with the incident beam,        an energy spectrum Si(E) is established for each pixel of the detector,        the energy spectra obtained earlier are combined.        
The gain in energy resolution obtained by virtue of the method and the device of FR11/62497 makes increasing the sensitivity possible, while keeping a very satisfactory energy resolution. To improve the sensitivity, it suffices to increase the opening of the detection collimator in order to collect more signal. But the observed field (or inspection volume, defined as being the volume, in the sample of materials, delimited by the incident beam and further delimited by the solid angle by which the detector sees the sample of materials—that is to say the solid angle delimited by the opening of the detection collimator and the detection plane of the detector —) is then greater, with an increased risk of superposition of materials, which may complicate the interpretation of the spectra obtained.